Fiber optic communications generally use modulated light sources, such as a laser, an optical receiver, notably a photodiode, and an optical fiber connecting the light source and the optical receiver. In this context, the laser emits pulses carrying data to be transported that will be received by the receiver, which converts these optical signals into electrical signals.
PIN structure photodiodes are generally used, which in particular comprise three layers: a p-doped electrical contact layer, an n-doped electrical contact layer and an intrinsic layer disposed between the two layers.
The p-doped and n-doped electrical contact layers are connected to respective metal contacts in the form of rings.
The intrinsic layer is in the form of a stud (also referred to as a mesa in the literature), for example in germanium, that absorbs incident light to convert it into a photocurrent or an electrical current.
Such a stud typically has a cylindrical shape of circular cross section and a diameter exceeding 10 μm in order to match the dimensions of the incident beam, which generally comes from an optical fibre.
The metal ring forming the metal contact in contact with the upper electrical contact layer generally has a diameter smaller than the diameter of the stud. Metal being opaque at the wavelengths concerned (near infrared: 850 nm to 1600 nm), this ring therefore masks some of the incident light, which reduces the performance/sensitivity of the photodiode.
To enhance the performance of such a photodiode, increasing the diameter of the stud of the intrinsic layer might be envisaged.
However, it is important that the diameter of the stud remain small, because it defines the capacitance of the photodiode and thus its bandwidth.
The smaller the diameter of the stud, the smaller the capacitance of the photodiode and the greater the bandwidth of the photodiode.
It would therefore be necessary to combine two mutually contradictory requirements, namely on the one hand to minimize the diameter of the mesa in order to optimize the bandwidth of the photodiode and on the other hand to maximize the diameter of the metal contact ring in order to optimize the sensitivity of the photodiode.
This problem has become particularly apparent with the broadening of the bandwidth to more than 10 GHz (i.e. 25 GHz, 40 GHz) although the diameter of the beam that illuminates the photodiode remains constant at 10-12 μm, being imposed by the standard for monomode fibres for optical telecommunication applications.
A partial solution to this problem is proposed in the document US2008/0265357.
As notably seen in FIG. 3 of that document, the upper electrical contact layer has a larger diameter than the intrinsic layer, which enables the use of a metal ring of larger diameter that no longer masks part of the stud of the intrinsic layer.
Although the performance/sensitivity of such a photodiode structure is undoubtedly improved, at the same time as retaining a high bandwidth, this approach ignores the appearance of stray capacitance that is added to the capacitance of the stud of the intrinsic layer and tends to limit the bandwidth of the photodiode.